1. Field of the Invention
The present invention relates to a complementary metal-oxide semiconductor (CMOS) image sensor and a method for manufacturing the same, and more particularly, to a CMOS image sensor and a method for manufacturing the same in which ions implanted in the vicinity of a device isolation film are prevented from being diffused into a photodiode region to reduce a dark current.
2. Discussion of the Related Art
An image sensor is a semiconductor device that converts optical images to electrical signals. The image sensor is classified into a charge coupled device (CCD) and a CMOS image sensor. The CCD stores charge carriers in MOS capacitors and transfers the charge carriers to the MOS capacitors. The MOS capacitors are approximate to one another. The CMOS image sensor employs a switching mode that sequentially detects outputs of unit pixels using MOS transistors by forming the MOS transistors to correspond to the number of the unit pixels using CMOS technology that uses a control circuit and a signal processing circuit as peripheral circuits.
The CMOS image sensor that converts data of an object into electrical signals includes signal processing chips having photodiodes. Each of the signal processing chips includes an amplifier, an analog-to-digital converter, an internal voltage generator, a timing generator, and a digital logic. Thus, it is economical in view of space, power consumption, and cost. The manufacture of the CCD requires technical process steps. However, the CMOS image sensor can be manufactured in mass production by a simple silicon wafer etching process cheaper than that used in the manufacture of the CCD. Also, the CMOS image sensor has an advantage in its packing density.
To display images, the CMOS image sensor sequentially detects signals in a switching mode by forming a photodiode and a transistor in a unit pixel. Also, since the CMOS image sensor uses CMOS technology, low power consumption is required. Furthermore, the number of masks required is less than the number of masks required for the CCD. For example, the number of masks for a CMOS image sensor is fewer by twenty than the thirty to forty masks required for the CCD. In this way, in the CMOS image sensor, process steps are simplified and various signal processing circuits can be integrated in one chip. Therefore, the CMOS image sensor has received much attention as an image sensor for the next generation.
FIG. 1 illustrates a unit circuit of a related art CMOS image sensor. The unit circuit of the CMOS image sensor according to a related art includes a photodiode and three MOS transistors. That is, the unit circuit of the CMOS image sensor includes a photodiode 1 generating optical charges using received light, a reset transistor 2 resetting the optical charges generated by the photodiode 1, a drive transistor 3 serving as a source-follower buffer amplifier, and a selection transistor 4 serving as an addresser.
As shown in FIG. 2, a P-type epitaxial layer (P-EPI) 11 is grown on a heavily doped P-type substrate 10, and a device isolation film 12 is formed in the epitaxial layer 11. The device isolation film 12 serves to isolate a lightly doped N-type photodiode region 16 from a device. A gate insulating film 14 and a gate electrode 15 of a transfer transistor are formed on the epitaxial layer 11. A heavily doped P-type diffusion region 13 is formed at both sides of the device isolation film 12 to prevent the occurrence of a dark current.
However, in the CMOS image sensor manufactured above, the dark current does occur. The dark current deteriorates performance of the image sensor and charges. The dark current is generated, even in case of no light, by electrons moving from a photodiode to a floating diffusion region. The dark current is caused by various defects, such as a line defect, a point defect, or dangling bond, occurring in the vicinity of a surface of a semiconductor substrate, between a device isolation film and a epitaxial layer, between a device isolation film and a photodiode, or in a P-type region and an N-type region. The dark current may cause a serious problem under a low illumination condition.
To reduce the dark current occurring in the vicinity of the surface of the semiconductor substrate, tilt ion implantation is performed in the lightly doped P-type epitaxial layer 11, the lightly doped N-type region 16, the heavily doped P-type substrate 10, and the device isolation film 12. However, in this case, impurity ions are excessively diffused into the photodiode region 16 by additional thermal diffusion performed when a well is formed.